RF power amplifiers are used in a variety of applications such as base stations for wireless communication systems etc. The signals amplified by the RE power amplifiers often include signals that have a high frequency modulated carrier having frequencies in the 400 megahertz (MHz) to 4 gigahertz (GHz) range. The baseband signal that modulates the carrier is typically at a relatively lower frequency and, depending on the application, can be up to 300 MHz or higher.
RF power amplifiers are designed to provide linear operation without distortion. RF transistors may have low input and output impedances (e.g., around 1 ohm or less for high power devices). Input and output impedance matching circuits are used to match the input and output impedances of the RE transistors to impedance matching networks from an external device, such as a circuit board.
A device package for an RE power amplifier can include a transistor die (e.g., MOSFET (metal-oxide semiconductor field-effect transistor), LDMOS (laterally-diffused metal-oxide semiconductor), HEMT (high electron mobility transistor), along with an input and output impedance matching circuit incorporated therein. The input and output impedance matching circuits typically include LC networks that provide at least a portion of an impedance matching circuit that is configured to match the impedance of the transistor die to a fixed value. The device package may also include tuning circuits that are configured to filter out higher order harmonic components of the fundamental frequency to improve amplifier efficiency.
A Chireix amplifier is one type of RF power amplifier that is gaining increased attention and popularity. The Chireix amplifier was first proposed by H. Chireix in 1935, and is described in “High power outphasing modulation,” Proc. IRE, Vol. 23, No. 11, pp. 1370-1392, November 1935, the content of which is incorporated by reference in its entirety. A Chireix amplifier utilizes an outphasing technique to amplify two phase-shifted constant envelope signals. The Chireix amplifier offers highly efficient and linear amplification without distortion. However, one source of inefficiency in a Chireix amplifier is attributable to the relative complexity of the output circuitry. Conventional Chireix amplifier designs that include impedance matching circuits and power combiner circuitry require a substantial number of components that can degrade efficiency and performance, and introduce substantial phase delay in the amplified signal.